1. Technical Field
In general, the invention relates to controlling the output features of electric power supplies. In particular, the invention relates to stabilizing the output voltage of a switched-mode power supply in a situation where load is not connected to the power supply.
2. Discussion of Related Art
Switched-mode power supplies are generally used in applications where the output voltage and output current of a device generating direct current should be controlled in an accurate and versatile manner. The principle of a switched-mode power supply is the feeding of electric power impulses to an inductive component, which has an energy-storing magnetic field. With a suitable discharge coupling, the stored energy is discharged into output voltage and output current, the values of which can be adjusted by altering the properties of the impulse supply. For the adjusting process, there are known several methods, and one of the most common is to measure the output voltage and/or output current of a power supply and to transmit the response signal describing the measuring result to a circuit that produces the switching pulses regulating the operation of the switching transistor.
FIG. 1 illustrates a simplified prior art switched-mode power supply 1 of the flyback type, including a double-coil transformer 2 for transmitting the electric power from the primary side to the secondary side, and an opto-coupler3 for transmitting the response signal from the secondary side to the primary side. The primary side receives its electric power from the AC source 4 and via the rectifier 5. The condenser 6 reduces interference in the rectified input voltage and prevents the high-voltage interference created by the switched-mode power supply from proceeding towards the AC source. A PWM (Pulse Width Modulation) controller 7 sends switching pulses to the base of the switching transistor 8, so that the primary current flows via the primary coil 2a during each switching pulse. On the secondary side, a diode 9 and a condenser 10 generate direct current to the output line 11 from the electric power transmitted to the secondary coil 2b, according to the known flyback principle (cf. for example Horowitz P., Hill W.: "The Art of Electronics", 2nd ed., ISBN 0-521-37095-7, Cambridge University Press, N.Y., USA, 1989, p. 355-368).
On the secondary side, a differential amplifier 12 compares the share of the output voltage generated by the voltage divider 13 with the voltage generated by the reference voltage generator 14. If the output voltage rises too high, a signal generated by the differential amplifier 12 lights up a light emitting diode provided in the opto-coupler 3, whereupon the transistor functioning as the other pair of the opto-coupler becomes conductive. Consequently, the simplified control principle of the switched-mode power supply 1 is that the primary side feeds a given maximum quantity of electric power to the transformer, until it receives from the secondary side a signal to the effect that the output voltage rises too high, whereupon the primary side begins to reduce the electric power supply. Often the switched-mode power supply also includes a current limiter arrangement, so that a value of the output current that rises too high causes a signal which is transmitted, via the opto-coupler 3 or via another signal coupler arranged in parallel thereto, to the primary side, where it limits the average electric power supplied into the transformer.
Let us next observe how the switched-mode power supply according to FIG. 1 is applied in a battery charger, i.e. in a device that is use or storing energy in a rechargeable electrochemical battery. Nowadays many portable electric devices are operated by batteries, and the structure and operation of said batteries requires that the charging current and voltage remain within given allowed limits. FIG. 2 illustrates, by way of example, a current-voltage graph, where the striped area 20 represents the range of the allowed current and voltage values. The horizontal axis of the graph represents current and the vertical axis represents voltage. When an empty battery is connected to the battery charger, and the charging current is switched on, the output voltage of the battery charger rises very rapidly from zero to a value describing the terminal voltage of the empty battery; in FIG. 2, this value is represented by the exemplary line 21. The charger must be provided with a current limiter which controls that the output current at this stage remains within given limits, i.e. within the striped area 20. When the battery begins to be charged, in the vicinity of the point 22 the charging current begins to decrease, because now the battery can only receive a small quantity of charging. The voltage limiter provided in the charger keeps the output voltage within the allowed limits between the points 22 and 23. If the battery is disconnected from the charger, but the charger remains switched on, its output voltage and output current are in theory set at the spot described by the point 23.
Known limiter arrangements generally have the drawback that they cannot maintain the output voltage of the switched-mode power supply constant, when the output current decreases very near to zero. In the current/voltage graph, this is shown so that when approaching the point 23 of FIG. 2, the output voltage rises near to the top limit of the allowed range, or even above it. This phenomenon is seen especially clearly in devices that can be adjusted for different output voltages. If we observe a battery charger that is based on a switched-mode power supply and is adjustable for output voltages 3-8 V, the rise of the output voltage with low values of the output current can be for instance 500 mV. Of an output current of eight volts, this constitutes only somewhat over 6%, and thus the share of the overvoltage is relatively small. On the other hand, of a three-volt output voltage, said excessive rise of 500 mV constitutes almost 17%, which already is a fairly large share. Overvoltage can damage a fully charged battery that is connected to a charger.